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Abstract

Boron tribalide complexes of 1,1-bis(dimethylamino)ethylene (DME) ,
t etramethylurea (TMU), tetramethylguanidine (TMG) , and pentamethylguanidine
(PMG) and also mixed boron t r ihalide adducts of DME have
been investigated by 1H and 19F NMR spectroscopy.
Both nitrogen and the C-Q-H carbon of DME are possible donor
a toms to boron trihal ides but complexation has been found to occur
only at carbon of DME. The initial adduct acts as a Bronsted acid
and gives up a proton to free DME in solut ion. A side reaction in the
DME-BF, system gives rise to trace amounts of a complex aSSigned as
(DME)2BF2+. (DME)2BF2+ is produced in much larger quantities in t he
DME-BF3-BC13 and DME-BF,-BBr, systems by reaction of free DME with
DME:BF2X (X = Cl, Br).
Restricted r otation about the C-N bonds of TMUlBC13 and n1U:BBr3
has been observed at low temperatures. This complements previous work
in this system and confirms oxygen donation of TMU to boron trihalides .
Restricted rotation at low temperatures also has been observed in DMEboron
trihalide systems